EP0271395A1 - Curved-bar probe for an echograph - Google Patents

Abstract

In the invention, a curved bar probe (1) for an ultrasound scanner is produced using a thin support (2) on which is placed a piezoelectric crystal bar. The bar is divided into a plurality of piezoelectric transducer elements (3). The thin support has the particularity of being rigid at room temperature but of being heat-deformable. By subjecting it to a heating-cooling cycle during which it is given a desired curved shape, one obtains at the end a rigid bar of the imposed curved shape. We can then avoid having to stick this support on a base (16). This would have the effect of constituting a rear reflection surface to the acoustic waves and creating parasites harmful to the useful acoustic signal.

Description

The present invention relates to a curved bar probe for an ultrasound system. It finds more particularly its application in the medical field where ultrasound scanners are used for diagnostic purposes to reveal images of internal tissue structures of human bodies studied. It can nevertheless find its application in all areas of use of ultrasound where curved bars are used.

An ultrasound system schematically comprises electrical generator means for producing an electrical signal vibrating at an acoustic frequency. This signal is applied to an element or rather to a strip of piezoelectric transducer elements where it is transformed into a mechanical excitation. The probe emits this mechanical excitation in a medium against which it is placed. Outside the transmission periods, the probe can be used to receive acoustic signals backscattered by the medium and to transform these acoustic signals into electrical signals that can be introduced into reception devices. From the electrical signal thus received, useful information can be extracted, in particular information likely to allow the creation of an image. The appearance of the image depends on how you explore the environment to be examined.

Among the various possible solutions, exploration by sectoral scanning is currently one of the most effective. To obtain it, it suffices to excite a group of adjacent elements in the bar, with predetermined delays relative to each other so as to focus the acoustic wave in a direction on emission (the same organization of delays is planned for reception to favor a signal from a given direction). By modifying the composition of the group of elements, by interrupting for example the supply of an element located on one side of the group and by putting into service another element located on the other side, and by reorganizing the delays for the new group thus formed, a new direction of focusing of the acoustic signals is obtained. If all the piezoelectric elements are aligned with each other along a straight line, the scanning of the medium studied is a scanning by translation. On the other hand, if the array of elements is curved, the scanning follows the normal to the tangent to the curve formed by the arrangement of the elements: it is sectoral if this curve is an arc of a circle.

The production of curved bar probes is traditionally carried out in the following manner. A support of relatively small thickness, for example 2 to 3 mm, made of a flexible material is used. Then a bar of a piezoelectric crystal is used which is fixed to this support. By cutting operations in particular with a saw, a partition is made in this bar so as to divide it into a plurality of piezoelectric elements. The partition is made in such a way that between each element the support is not cut. Each element remains attached to the support. As the support is flexible, it suffices to fix it on a suitable curved base to obtain a desired curved bar. In European Patent Application ^No. 84 308 373.4 filed Dec. 3, 1984, such an embodiment is described.

However, this way of doing things has a drawback. In fact, due to the elastic nature of the support, it can only be kept curved by exerting a permanent support force. As indicated in the cited document, this effort can be obtained by bonding the support to the base. The face of the piezoelectric elements facing the place where the useful acoustic waves propagate is called the front face, the face opposite to the front face is called the rear face. During the emission, the acoustic wave emitted propagates a priori in both directions: towards the front and towards the rear. Only the wave transmitted forward is useful. We manage accordingly to avoid disturbances due to the rear wave. In particular, the acoustic impedance of the support and of the base to prevent the rear wave from being reflected towards the bar. However, this can only be imperfectly obtained from the moment when the support has a bonding surface on the base. This bonding surface reacts all the more strongly the thinner the support in order to be able to be deformed and therefore that this surface is located near the piezoelectric strip proper.

Despite the changes in composition of the glue used to secure the support to its base, as indicated in the cited document, the best result is not obtained. In addition, the bonding operations are never perfect and the solidity of the assembly can be achieved. Finally, the very nature of the flexible materials which can be used for this purpose is not conducive to the choice, for the support material, of a good acoustic impedance. It is known in fact to play on the acoustic impedance of materials by making mixtures or by including microbeads in them, for example plastic or glass. And flexible synthetic materials do not lend themselves well to this operation.

The object of the invention is to remedy the drawbacks mentioned by proposing for the support a material which is rigid at room temperature, but which has the particularity of being heat-deformable. This means that if this material is heated, it can be deformed. When it cools, it keeps the shape acquired last: that which was given to it when it was hot. As this heat-deformable material retains the shape that it has been given, it is possible to use it as a receptacle for pouring therein a material to polymerize which will serve as a base. The polymerization preferably takes place at room temperature. The material to be polymerized is preferably the same material as that which serves to make the support. As a result, there is no longer a surface creating parasitic reflections from the rear wave. Finally, rigid heat-deformable materials also have the advantage of being able to be easily adapted to acoustic impedance.

The present invention relates to a curved strip probe for an ultrasound system, of the type comprising piezoelectric elements fixed on a deformable support, characterized in that the support is made of a rigid heat-deformable material at ambient temperature.

• - figure 1 une barrette d'éléments piézo-électriques montés sur un support conforme à l'invention;
• - figure 2 l'allure de la barrette précédente après déformation con­sécutive à un cycle d'échauffement suivi d'un refroidissement.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. They are given only as an indication and in no way limit the invention. In the figures, the same references designate the same elements. The figures show:

• - Figure 1 a bar of piezoelectric elements mounted on a support according to the invention;
• - Figure 2 the shape of the previous bar after deformation following a heating cycle followed by cooling.

FIG. 1 represents a bar of piezoelectric transducer elements mounted on a support according to the invention. This bar 1 essentially comprises a support 2, piezoelectric elements such as 3, these piezoelectric elements being held between the support 2 and acoustic transition blades 4. Between the support 2 and each of the elements 3 there is an electrode 5, and between each of the elements 3 and each of the blades 4 there is an electrode 6. These electrodes are intended to receive an electrical signal at the time of an excitation. They then apply in element 3 a corresponding electric field. Under the effect of this field, the element 3 begins to vibrate and transmits, by the blade 4, the vibraton to a medium to be studied and which is in contact with it (not shown). In a preferred embodiment, the electrical continuity of the electrodes 5 and 6 is taken up by blocks such as 7 and 8 placed on either side of each element. Each block of insulating material is covered with two electrically independent metallizations, respectively 9 and 10, in each case in contact with the electrodes 5 and 6. These metallizations allow a simpler connection to the electrodes 5 and 6.

Except for the presence of blocks 7 and 8, the bar 1 is manufactured in the rectilinear state as in the cited state of the art. An elongated support 2 is used to construct it, a bar of material piezoelectric, and an elongated blade: the support and the blade are glued to the bar. For the connection improvement mentioned, during assembly, strips on both sides of the bar are already fitted with the metallization partition. When this assembly is formed, cuts such as 11 and 12 are made, in general with a saw, to divide the piezoelectric bar into a series of independent piezoelectric elements. The support is only partially started by these cutouts: it ensures the cohesion of all the elements. It is known to avoid risks of crosstalk between adjacent piezoelectric elements to divide each piezoelectric element (separated from each other by deep cutouts 12), by shallow cutouts 11 which cut them in the middle.

The essential characteristic of the invention resides in the nature of the material which constitutes the support 2. When the cutting operation is finished, even the deepest cuts 12 do not break its rigidity. The bar remains straight and rigid: the material of which it is made is hard. This material, however, has the particularity of softening when heated and taking the shape that is imposed on it at that time. An appropriate curved shape 13 is therefore used, and the shape 13 and the strip 1 are placed in an oven which is brought to an adequate temperature. Under the effect of its own weight, or possibly by exerting an elastic force on its ends 14 and 15, it is possible to cause the bar to bend according to the shape of the shape 13. After a time judged experimentally long enough, the oven is allowed to cool. The bar then has the appearance shown schematically in Figure 2: it is now again rigid but curved.

In a variant, the thermoformable curved support is constituted by the acoustic transition blade itself. When dividing the bar, sawing is carried out from base 2 to a certain height in the blade. In either case, it is subsequently possible, by making saw cuts of appropriate width, to form convex or concave curved bars.

The material which can be used for the support 2 and which has the thermoforming properties thus demonstrated is preferably a polymerizable material which has the appearance of a foam before its polymerization. This foam can be syntactic, that is to say comprise a liquid with gas microbubbles, or be non syntactic, that is to say appear in the form of beads which agglomerate with each other during the polymerization. This foam is preferably an epoxy resin or else a polyurethane. It is preferably a cold polymerizable foam. To adapt the impedance of the support, provision is made to load the foam with microbeads, in particular plastic microbeads. In a preferred example, the plastic microbeads are the phenolic microbeads. In practice, materials are chosen whose thermoforming is obtained at a temperature of the order of, or greater than 90 ° - 100 °. In fact, it is known that piezoelectric probes heat up during their use. They are then brought to temperatures which could, if we were not careful, cause fluence unwanted deformations of the bar. For this reason, the thermoforming temperature is chosen at the value indicated. At this temperature, in fact, the probes cannot be used on human bodies, and there is therefore no risk that this temperature will be reached during an experiment.

To allow its thermoforming, it is necessary, as in the state of the art, that the support 2 is thin. In the invention it has a thickness substantially equal to the elastic support of the cited state of the art. To then reinforce the rigidity of this support, it is secured to a base 16. Then the thermoformed bar is placed at the bottom of a mold, with its concave part upwards, and the same material is poured from above as that who made up the support (but not yet polymerized). Then the base material is polymerized: the mold is shaped so as to give this base, moreover, a form useful for handling the probe. The base is made as soon as possible after the thermal deformation of the bar. For example, this operation is done the next day.

As the material which constitutes the base is the same as that which constitutes the support, if these operations are well executed, at the end, it is almost impossible to discern the part, in the support-base, of what belongs to the support or at the base. There is therefore no longer any acoustic reflection surface under the support. Reflections can therefore no longer occur. The advantage of having chosen a cold polymerizable material is easily understood. During the subsequent production of the base, it is not necessary to polymerize this base material to heat the entire strip. This would risk destroying the deformation we gave it previously. The choice of a hot-polymerizable material is however possible: it suffices to pour the base before the end of the thermo-deformation operation, that is to say before cooling. The advantage of thermo-deformable materials, is also to be able to accept a wide variety of loading materials. This gives them a great ability to correctly adjust the acoustic impedance.

To make the connection of the piezoelectric elements of the strip, it is possible to deposit on the apparent lateral metallizations blocks of each element of micro-drops 17 of Indium. Then, on each side of the bar, a printed circuit 18 provided with connection tracks such as 21 and 22 is approached. This circuit bears, opposite the connections to be metallized 19, also provided with micro-drops 20 of Indium. The printed circuits are then placed on the blanks of the strip and, by a reflow operation in an oven, it is possible to obtain the connection of all the elements to the tracks. These tracks conduct the electrical signals, on transmission and on reception, from the generating members and to the receiving members.

The shape presented so far for the bar is a convex shape. However, it is known in the state of the art to manufacture concave shapes. It is obvious that a concave bar probe can be made in the same way. The only precaution to take consists in making in this case cuts 10 and 11 sufficiently wide so that during the deformation imposed on the bar the piezoelectric elements do not come into contact with each other. In this case also, a base can be produced which adheres intimately to the support.

Claims (10)

1 - Curved strip probe (1) for an ultrasound system of the type comprising piezoelectric elements (3) fixed on a deformable support (2) characterized in that this support is made of a rigid thermo-deformable material at room temperature. 2 - Probe according to claim 1, characterized in that the material is a cold polymerizable epoxy resin. 3 - Probe according to claim 1, characterized in that the material is a polyurethane. 4 - Probe according to any one of claims 1 to 3, characterized in that the material is a foam. 5 - A probe according to claim 4, characterized in that the foam comprises plastic microbeads. 6 - Probe according to claim 5, characterized in that the microbeads are phenolic. 7 - Probe according to any one of claims 1 to 6, characterized in that the heat-deformed support is fixed to a base (16) which matches its shape and which is made of the same material. 8 - Probe according to any one of claims 1 to 7 characterized in that the bar is convex curve (fig 2). 9 - Probe according to any one of claims 1 to 7, characterized in that the bar is curved and concave. 10 - Method for manufacturing a curved strip probe (1) characterized in that it comprises the steps of:
bonding of a piezoelectric strip on a heat-deformable support blade (2),
making of cuts (11, 12) making it possible to produce independent piezoelectric transducers and / or to divide the piezoelectric transducers,
- heating and deformation of the bar to give it the desired shape,
- connection of the electrodes to the piezoelectric transducers.

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